Multi-wall sheets are often provided where one or both of the outer coextruded walls are capable of performing any of a variety of functions. For example, one or both outside walls may provide UV protection for the sheet against damage (e.g. yellowing) by UV radiation, but other functions, e.g. IR reflection, are also performed in this way.
The state of the art relating to multi-layer products is summarized below.
EP-A 0 110 221 discloses sheets consisting of two layers of polycarbonate, one layer containing at least 3 wt. % of a UV absorber. These sheets may be produced by coextrusion according to EP-A 0 110 221.
EP-A 0 320 632 discloses moldings consisting of two layers of thermoplastic, preferably polycarbonate, one layer containing a specially substituted benzotriazoles as UV absorbers. EP-A 0 320 632 also discloses the production of these moldings by coextrusion.
EP-A 0 247 480 discloses multi-layer sheets in the structure of which includes a layer of branched polycarbonate and a layer of thermoplastic, the polycarbonate layer containing a specially substituted benzotriazoles as UV absorbers. The production of these sheets by coextrusion is also disclosed.
EP-A 0 500 496 discloses polymer compositions stabilized against UV light with special triazines, and their use as the outer layer on a multi-wall sheet. The polymers mentioned are polycarbonate, polyester, polyamides, polyacetals, polyphenylene oxide and polyphenylene sulfide.
However, all the coextruded multi-wall sheets known from the state of the art exhibit the so-called “triangle effect”, a defect resulting upon the coextrusion, described as a combination of gussets consisting of the material making up the coextruded layer with unevenness of the surface of the sheet (FIG. 1).